The present invention is concerned generally with methods of doping a silicon wafer with phosphorus and subsequently growing an oxide on the doped silicon wafer. More particularly, the present invention is concerned with a method of doping a silicon wafer with phosphorus using a solid dopant source and then growing an oxide layer on the surface of the doped silicon wafer in pyrogenic steam or in a wet oxygen environment in the presence of the solid phosphorus source to accomplish an in situ oxidation of the doped silicon surface. As used herein, xe2x80x9cin situ oxidationxe2x80x9d means oxidation of the doped silicon in the presence of the solid dopant sources.
Several conventional methods of doping silicon wafers exist. In one of these methods, the silicon wafer is first doped at a high temperature, usually ranging from about 950-1150xc2x0 C., using phosphorus oxychloride (POCl3) or phosphine (PH3). The gaseous POCl3 or PH3 is carried down the diffusion tube with nitrogen containing some oxygen. This results in the deposition of phosphorus oxide on the silicon surface. The phosphorus oxide then reacts with the silicon surface to form elemental phosphorus, which in turn diffuses to a controlled depth within the silicon wafer. Immediately following the deposition is a steam oxidation of the doped silicon wafer, usually near the deposition temperature, for about 30-60 minutes to grow the desired oxide. Generally, the concentration and the depth of the diffused phosphorus within the silicon wafer are proportional to the time and temperature of the entire deposition process. The thickness of the resulting oxide layer formed on the surface of the silicon wafer during the steam oxidation step is proportional to the time and temperature in steam.
There are a number of difficulties associated with the use of POCl3 and PH3. These include non-uniform doping of large diameter silicon wafers (because the dopant gas cannot easily get between the silicon wafers), generation of particles in the diffusion tube that cause silicon damage, toxicity and corrosivity of the gases, and other handling difficulties.
Solid dopant sources that evolve phosphorus pentoxide (P2O5) upon heating, such as those described in U.S. Pat. No. 3,998,668 to Florence et al., were developed to overcome some of the problems associated with gaseous dopants. The P2O5 is deposited on the silicon wafers during the deposition step. U.S. Pat. Nos. 4,175,988, and 4,141,738 to Rapp, the contents of which are incorporated herein by reference, describe exemplary solid dopant sources and methods of using the solid dopant sources to dope a silicon wafer.
One method of doping silicon wafers using solid dopant sources involves doping the silicon wafer at about 1050xc2x0 C. followed by an in situ oxidation with dry oxygen at about the same temperature. During the deposition step, the silicon wafers are placed with their surfaces parallel and in close proximity to the surfaces of adjacent solid dopant sources, e.g., in a solid dopant source boat. The solid dopant sources and silicon wafers are inserted into a diffusion tube and heated to the deposition temperature under nitrogen using a conventional furnace ramping technique. The oxidation of the doped silicon wafers can be done in situ at temperatures near 1050xc2x0 C. because dry oxygen does not change the evolution rate of the sources.
When silicon wafers are doped using solid dopant sources followed by oxidation with dry oxygen, much more time is required to produce the desired oxide thickness because oxygen oxidizes the silicon more slowly than steam. This longer oxidation time at about 1050xc2x0 C. also results in excessive diffusion of phosphorus into the silicon wafer. Unless the desired junction is very deep, the additional diffusion time results in too deep of a junction and often destroys the device being made.
Another method of doping silicon using solid dopant sources involves doping the silicon wafers at about 1050xc2x0 C., removing the silicon wafers from proximity to the solid dopant sources, and oxidizing the silicon wafers with steam. After deposition, the silicon wafers are removed from the boat (and proximity to the solid dopant sources) and re-inserted into the diffusion furnace for the steam oxidation. The separate steam oxidation can be done at temperatures similar to those in the first method described above to grow the desired oxide. The solid dopant sources must be isolated from the silicon wafers during steam oxidation because moisture at high temperatures causes P2O5 to evolve rapidly from the source. This method yields satisfactory results but is time consuming and requires much operator handling. It also exposes the hygroscopic silicon surface containing P2O5 to humid room air, which can produce silicon surface damage.
The thickness of the oxide layer desired on a silicon wafer may vary with the application from a few hundred angstroms (xc3x85) to 20,000 xc3x85 or more. In most emitter diffusions, the oxide thickness will be in the range of about 1500-6000 xc3x85 and preferably in the range of about 2000-5000 xc3x85. A typical emitter diffusion will require an oxide thickness of about 4300-5000 xc3x85. Oxide layers of the preferred thickness for emitter diffusions typically are grown at or near the deposition temperature, which typically is in the range of about 950-150xc2x0 C. Acceptable sheet resistivities for emitter diffusions typically are in the range of about 5.35-6.15 ohms/sq. but may be as high as about 15 ohms/sq. and as low as about 1 ohm/sq. For example, a deposition temperature and time of around 1025xc2x0 C. for 60 minutes commonly is used for bipolar transistors to give sheet resistivities of about 5 ohms/sq.
It is an object of the present invention to provide a method for doping a silicon wafer using a solid phosphorus dopant source in which the doped wafer is oxidized while in close proximity to the solid source in situ while maintaining the useful life of the source.
It is a further object of the present invention to provide a method for obtaining a satisfactory oxide thickness layer on a phosphorus-doped silicon wafer at a lower temperature than conventional oxidation methods without excessively long oxidation times, and particularly a method for obtaining an oxide layer of any desired thickness by selection of the appropriate wet oxidation time at the lower temperature.
It is another object of the invention to provide a method for oxidizing a phosphorus-doped silicon wafer that maintains the diffusion of phosphorus into the silicon at a depth comparable to that observed when using doping methods such as POCl3 or PH3 that use conventional wet oxidation methods at elevated temperatures.
It is yet another object of the invention to provide a method for doping and oxidizing a silicon wafer that reduces handling of the silicon wafers and exposure of the silicon wafers to humid room air compared to conventional wet oxidation methods using solid dopant sources.
These and other objects of the present invention will be apparent from the specification that follows, the appended claims, and the drawings.
The present invention overcomes the problems associated with conventional solid source doping methods. It allows doping of silicon wafers with a solid phosphorus dopant source followed by a steam oxidation of the doped silicon wafers without first removing the wafers from the diffusion furnace and without drastically decreasing the life of the solid dopant sources.
The method of the present invention involves placing a silicon wafer in spaced relationship to a solid phosphorus dopant source at a first temperature for a time sufficient to deposit a phosphorus-containing layer on the surface of the wafer, oxidizing the doped silicon wafer by a wet oxidation method at a second temperature lower than the first temperature, and maintaining the silicon wafer in spaced relationship with the solid phosphorus dopant source during the oxidizing step. The first and second temperatures are selected such that the solid phosphorus dopant source evolves P2O5 at the first temperature and the second temperature is sufficiently lower than the first temperature to decrease evolution of P2O5 from the solid phosphorus dopant source during the oxidizing step. The second temperature may be selected to be less than or equal to about 900xc2x0 C. and particularly in the range of about 600 to 900xc2x0 C.
In the present method, the step of maintaining the silicon wafer in spaced relationship to the solid phosphorus dopant source may include placing the silicon wafer in close proximity and substantially parallel to the solid phosphorus dopant source, with the silicon wafer being maintained in this position during the oxidizing step. The temperatures are selected such that the solid phosphorus dopant source evolves P2O5 at the first temperature and the second temperature is sufficiently lower than the first temperature to decrease evolution of P2O5 from the solid phosphorus dopant source during the oxidizing step. The second temperature may be selected to be in the range of about 600 to 900xc2x0 C.
The method further may include the step of selecting a wet oxidation method from oxidization with wet oxygen and oxidation with pyrogenic steam, in which oxidation with wet oxygen includes oxidation with oxygen bubbled through deionized water. The method also may include the step of selecting a solid dopant source that decomposes at high temperatures, the solid dopant source being selected from an aluminum metaphosphate (Al(PO3)3) dopant source, a rare earth metaphosphate (Ln(PO3)3, where xe2x80x9cLnxe2x80x9d refers to a rare earth element) dopant source, and a rare earth pentaphosphate (LnP5014) dopant source.
The method of the invention may include the step of maintaining a silicon wafer and a phosphorus solid dopant source for vapor phase deposition of P2O5 in vapor phase communication at a temperature and for a time sufficient to form a layer of n-type conductivity on the surface of the silicon wafer and subsequently oxidizing the doped silicon wafer with an oxidizing agent selected from wet oxygen and pyrogenic steam at a second temperature for a time sufficient to form on the surface of the silicon wafer an oxide layer having a predetermined thickness, with the second temperature being sufficiently lower than the first temperature to decrease the evolution rate of P2O5 from the solid dopant source while the silicon wafer is maintained in vapor phase communication with the solid dopant source during the oxidation step. The second temperature may be selected to be in the range of about 600 to 900xc2x0 C. For example, the oxidation step may be carried out at about 775xc2x0 C. for about 3 hours to form on the surface of the silicon wafer an oxide layer having a thickness of about 4000 xc3x85. The method may include the step of selecting a solid dopant source from an aluminum metaphosphate dopant source, a rare earth metaphosphate dopant source, and a rare earth pentaphosphate dopant source.
The invention also provides a method of doping a silicon wafer including the steps of maintaining a silicon wafer and a solid phosphorus dopant source in close proximity for vapor phase deposition of P2O5 at a temperature and for a time sufficient to form a layer of n-type conductivity on the surface of the silicon wafer, oxidizing the doped silicon wafer with an oxidizing agent selected from wet oxygen and pyrogenic steam at a second temperature lower than the first temperature, and maintaining the silicon wafer in close proximity to the solid phosphorus dopant source during the oxidizing step. The second temperature, which is selected to be sufficiently lower than the first temperature to substantially decrease the evolution of P2O5 from the solid dopant source during the oxidizing step, may be in the range of about 600 to 900xc2x0 C.
The oxidizing step may be carried out at the second temperature for a time sufficient to form on the surface of the silicon wafer an oxide layer of a predetermined thickness, such as a thickness of about 4000 xc3x85. The method also may include the step of selecting a solid dopant source that decomposes at a high temperature, the dopant source being selected from an aluminum metaphosphate dopant source, a rare earth metaphosphate dopant source, and a rare earth pentaphosphate dopant source.
The method of doping a silicon wafer may include the steps of placing a silicon wafer in spaced relationship to a solid phosphorus dopant source at a first temperature for a time sufficient to deposit a phosphorus-containing layer on the surface of the wafer, oxidizing the doped silicon wafer by a wet oxidation method at a second temperature lower than the first temperature, and maintaining the silicon wafer in spaced relationship to the solid phosphorus dopant source during the oxidizing step. The temperatures are selected such that the solid phosphorus dopant source evolves P2O5 at the first temperature and the second temperature is sufficiently lower than the first temperature to decrease evolution of P2O5 from the solid phosphorus dopant source during the oxidizing step. The second temperature may be selected to be less than or equal to about 900xc2x0 C. and particularly in the range of about 600 to 900xc2x0 C.
The above-described method may include one or more of the steps of maintaining the silicon wafer in close proximity and substantially parallel to the solid phosphorus dopant source during the oxidizing step and selecting a wet oxidation method from oxidation with wet oxygen and oxidation with pyrogenic steam. The temperatures are selected such that the solid phosphorus dopant source evolves P2O5 at the first temperature and the second temperature is sufficiently lower than the first temperature to decrease evolution of P2O5 from the solid phosphorus dopant source during the oxidizing step. The second temperature may be selected to be in the range of about 600 to 900xc2x0 C. The method further may include the step of selecting a solid dopant source from an aluminum metaphosphate dopant source, a rare earth metaphosphate dopant source, and a rare earth pentaphosphate dopant source.
The invention further provides a method of growing an oxide layer on a silicon wafer, including the step of wet oxidizing a doped silicon wafer at a temperature sufficient to substantially decrease the P2O5 evolution rate of a solid phosphorus dopant source maintained in vapor phase communication with the silicon wafer during the wet oxidizing step. The wet oxidizing temperature may be selected to be less than or equal to about 900xc2x0 C. and particularly in the range of about 600 to 900xc2x0 C. The wet oxidizing step may involve exposing the doped silicon wafer to an oxidizing agent selected from wet oxygen and pyrogenic steam.
The method may include the step of selecting a temperature and time for the wet oxidizing step sufficient to form on the surface of the doped silicon wafer an oxide layer having a predetermined thickness. The wet oxidizing temperature may be selected to be in the range of about 600 to 900xc2x0 C. For example, the wet oxidizing step may be carried out at about 775xc2x0 C. for about 3 hours. The method also may include the step of forming a doped silicon wafer by heating a silicon wafer in vapor phase communication with a solid dopant source. The solid dopant source may be selected from an aluminum metaphosphate dopant source, a rare earth metaphosphate dopant source, and a rare earth pentaphosphate dopant source
These and further objects of the invention will become apparent from the following detailed description.